A power supply or power converter converts one form and voltage of electrical power to another desired form and voltage. AC-to-DC power supplies convert alternating current voltage, for example 115 or 230 volt alternating current (AC) supplied by a utility company, to a regulated direct current (DC) voltage. DC-to-DC power supplies convert DC voltage at one level, for example 400V, to another DC voltage, for example 12V.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. An SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage.
A SMPS uses a high frequency switch, a transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter. SMPSs can be used to step-down a supply voltage as well as provide a step-up function and an inverted output function. An SMPS converts an input voltage level to another level by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The storage may be in either electromagnetic components, such as inductors and/or transformers, or electrostatic components, such as capacitors. A load is coupled to the output to utilize the delivered energy. When no load is coupled to the output of the power converter, a no load power condition exists. No load power consumption is the energy consumed by the power converter when the power converter is coupled to an input power supply but no load is connected, such as when a power charger is plugged into a wall socket but without a mobile phone or other handheld device being connected.
With the introduction of high speed composite semiconductor switches, such as metal oxide semiconductor field effect transistor (MOSFET) switches operated by pulse width modulation (PWM), recent SMPS topologies are now capable of operation at greatly increased switching frequencies, such as, for example, up to 1.0 MHz. However, to minimize the no load power of a SMPS it is necessary to minimize the switching frequency and the amount of power that is transferred to the secondary side per pulse.
The power that is transferred to the secondary side is P=Wp*fs, where Wp is the energy transferred with each pulse and fs is the switching frequency of the main switch. To minimize the no load power the transferred power P must be as small as possible because it must be consumed by a base load. Otherwise the output voltage rises if no load is connected. The pulse must have a minimum pulse width to ensure that some energy is transferred to the secondary side. To minimize the transferred power P it is necessary to decrease the energy Wp that is transferred with each pulse and the switching frequency fs.
Modern SMPS controllers control the output voltage and current by measuring the reflected voltage at a winding of the transformer. If the transferred energy per pulse is very low, then it is very difficult to detect the output voltage because the reflected pulse is influenced by parasitic effects. If the frequency is very low, then a change of the output voltage can only be detected with a big delay. Therefore fast response to a load change is not possible.